Using extant data, compare only-children and children with siblings enrolled in the Kaiser Health Plan. Records will be analyzed of the children at ages five, ten, and fifteen to seventeen years of age. Analysis will include such information as the mother's reaction to pregnancy, the child's physical growth, cognitive abilities, behavioral characteristics, mother's contraceptive use, and "intendedness" of future children. Consequences of being an only child (compared to being oldest and youngest in two and three child families) will be analyzed and determinants of the one child family will be described. Racial differences will be assesses as is feasible.